The invention relates to a coxofemoral prosthesis comprising an implant intended to be engaged in the medullary cavity of the femur for replacement of both parts of the articulation which has failed for pathological reasons or because of an accident. It is known that a major problem to be solved in such cases is in anchoring of the prosthesis in the femur.
For a little over thirty years, two anchoring techniques have been used, namely: anchoring with a surgical cement, for example, with a methyl methacrylate base; and anchoring without cement, in which case locking is obtained by osteogenesis in contact with the implant in the femoral metaphysis.
It is easy to understand that on the plane of transmission of forces these two techniques are fundamentally different, even though they bring into play structures having certain resemblances between them.
This invention aims at a type of coxofemoral prosthesis of the type not using cement.
These are numerous patents relating to this type of prosthesis. There is found in the prior art the description of implants, often called "pins", more or less curved to follow the axis of the femur in which they are implanted. See, for example, French Pat. Nos. 2,242,065, 2,194,123, 2,528,307, 2,299,012 and 80,495; also West German Pat. No. 3,003,050.
The tail of the implant is either smooth for locking with cement (e.g., No. 2,299,012), or partially smooth (No. 2,528,307) or grained (No. 2,194,123) and generally of circular section, but able to have a cruciform section (No. 80,495) or polygonal section (No. 2,528,307 or No. 2,419,717). Tails are also known provided with a plurality of annular projections like that of an anchor bolt. For example, in No. 2,242,065 or No. 2,295,730, projections are provided perpendicular in relation to the axis of the implant and are intended, in the mind of their inventor, to be perpendicular to the loading force.
Implant prostheses screwed into the medullary cavity can also be cited. It should be noted that in all those cited above, the locking is performed by application of a plate perpendicular to the axis of the implant, on the stump part of the great trochanter or on the cervicotrochanterian fracture as in the case of No. 2,528,307.
Clinical observations have shown that prostheses with a slightly curved tail, plate and projections perpendicular to the axis of the implant were not free from drawbacks and in no way met the expectations of their inventor. We can point out a course failure after short or long term, and a bad distribution of pressure.
The curved profile itself may produce a varus shifting of the tail, driving at higher pressure on the internal cortical (frontal plane) and almost a complete loss in the sagittal plane, and a possibility of fracture of the tail or of internal shifting with cortical reaction at its level.
The poor orientation of the projections can result in microfractures of the osseous neoformations able to cause the implant to migrate, either by sinking in or by releasing it with a painful prosthesis as a consequence.
Further, it should be noted that the presence of crosswise projections make an extraction very difficult in case of a later operation.
The implant according to this invention is conceived according to totally different principles to avoid the above drawbacks. In the present invention, the tail has a straight axis and is slightly conical in shape, and it comprises neither base nor projections perpendicular to this axis but on the contrary, has projections inclined from the internal side to the external side approximately along the axis of the conical lug carrying the ball-and-socket head. These projections, in addition, have a lance shape, i.e., hardly projecting on the internal side, greatly projecting on the anterior and posterior faces, and tapering to a simple tip in the extension of the external cortical wall.
Plainly, the absence of the support plate at the level of the upper part of the prosthesis, on the one hand, and the presence of oblique projections on both the anterior and posterior faces, on the other hand, enable the implant itself to find its place.
Actually, during placing of the prosthesis, these projections guide and orient its sliding, and in the longer term, the spongy bone which is carried and packed against the inside of the cortical wall is transformed into osseous bars which serve as additional support and thus reduce the pressure at the level of the internal cortical wall.
It should also be noted that these osseous bars have the overall direction of force lines of the anterior and posterior cortical walls of the normal femur.
Thus, the artificial support duplicates the natural bone structure in the most anatomical manner possible. In other words, the originality of this prosthesis is, on the one hand, to be an implant whose sliding is controlled by the profile that it exhibits and, on the other hand, to be locked in good position because of its tulip shape both in the frontal plane and in the saggital plane.
It should be noted that this profile fills the available space in the upper part of the femur to be connected, as precisely as possible, along the cortical walls of the upper femoral methaphysis.
Finally, because of its profile, to limit sliding and excessive sinking during placing and impaction of the prosthesis, the internal edge of the latter is more concave than the corresponding femoral cortical wall. Thus, any attempt at excessive impaction will rest on the internal part of the prosthesis which then can only undergo a translation toward the external cortical wall, putting the lance-shaped projections of the external edge of the prosthesis in contact with the external cortical wall of the femur.
All these arrangements contribute, of course, to an extremely powerful blocking especially of possible rotational movements, already greatly limited by the general shape of the upper end of the implant.